Ion exchange resins include both gel type and macroporous resins. Generally, gel type resins consist of transparent, glassy beads having good breaking weights and high volume capacities allowing for good flow velocity through tall column beds containing the gel beads.
One advanced method of forming gel resins involves the use of interpenetrating polymeric networks (IPN's). IPN's were first conceptualized by John Millar. IPN's are defined as macromolecular assemblies comprising two or more polymers where at least one is in the form of a network; the polymers are at least partially interlaced on a molecular scale although not covalently bonded to each other. Because there is no chemical bonding between the networks (or polymer/network), each may retain its individual properties independently of its individual proportion in the blend material. As a result, improvement can be attained in properties such as mechanical strength, impact resistance, and toughness. There are two main types of IPN's, semi-IPN where at least one component is not in network—i.e., crosslinked—form, and full IPN's where all species are in network form.
The initial experiments to form IPN gels as described by Millar started with a crosslinked styrene-divinylbenzene (DVB) gel copolymer which was re-swollen with fresh styrene-DVB mixtures and the polymerization carried out for a 2nd time (1st order IPN), and optionally a 3rd time (2nd order IPN), etc. The solvent swelling of the subsequent copolymers was less than that expected for the actual, algebraic amount of DVB present, leading to the conclusion of enhanced entanglement of the polymer networks. Indeed, from this observation, “simple” gel beads themselves, appear to be 0th order IPN's.
Although Millar himself was concerned with the use of IPN resins in the field of ion-exchange, many other studies on IPN's have been carried out in other areas, such as the extensive work of Sperling et al. (see Sperling, Polym. Eng. Sci., vol. 25, No. 9, pp. 517-520, 1985 and Sperling “Interpenetrating Polymer Networks and Related Materials,” pp. 202-204, 243-261, (Plenum Press, 1981)).
There are currently several methods of producing IPN's of gel networks. U.S. Pat. No. 5,231,115 describes the use of gel-type copolymer seed particles and monovinylidene monomers used to produce IPN's. U.S. Pat. Pub. 2002/01222946 provides IPN's formed from silicone oligomers and silsesquioxane oligomers. U.S. Pat. Pub. 2003/0000028 provides an IPN in tinted contact lenses. U.S. Pat. Pub. 2006/0148985 provides get IPN from silicone polymers where CO2 is used as a solvent. U.S. Pat. Pub. 2002/0052448 discloses forming a first network and thereafter swelling this network with monomers and cross-linking agents to form an IPN. Other IPN's are formed from simultaneously forming and crosslinking the polymer networks.
Barrett et al. in U.S. Pat. No. 4,582,859 found enhanced swellings when forming an IPN where the swelling was over the limit of the starting copolymer. Therefore, further issues, than just the enhanced entanglement found by Millar, need to be considered.
However, gel beads offer poor performance in applications demanding stability to osmotic changes as well as where the resin swells or shrinks greatly, due to changes in ionic form. Gel beads can also be poor performers where bead diffusion is used for the separation/absorption process due to a slow rate of diffusion into the resin beads for the chemical species being separated. To optimize separation capacity for the resin, all available exchange sites within the resin bead volume should be readily available for exchange. This involves using the entire available diffusion path length, which corresponds to the gel bead radius for a fully functionalized resin bead. A relatively long time is required to reach exchange equilibrium when using the full path length due to the limiting rate of diffusion through the beads. Beads that reach exchange equilibrium more rapidly and allow for more rapid access to available exchange sites can be made by reducing the diffusion path length (i.e., making beads with a smaller radius/diameter). However, small beads lead to larger pressure drops with a resin bed, reduced flow rates for feed streams being processed, and other problems related to the handling of fine beads.
Macroporous resins, such as those described by Abrams and J. R. Millar (React. Funct. Polym. 35 (1997), pp. 7-22) and in U.S. Pat. No. 4,224,415, were developed to improve kinetics by providing a highly porous copolymer bead matrix for ion exchange wherein relatively large pore sizes improves diffusion of chemical species into the interior portions of the beads (i.e., ion exchange resins, or IEX resins). Macroporous resins contain significant non-gel porosity in addition to normal gel porosity. This non-gel porosity arises from channels present between the gel lattices. These microscopic channels are separate and distinct from the micropores, which are present in all crosslinked IEX resins, as is well known to those skilled in the art. While the channels are themselves relatively small, they are large when compared with the micropores of the previously known, gel type resins. IEX resins generally have bead diameters within about 150-1,200 μm.
Various macroporous resins and methods for generating macroporosity are known in the art. The terms “macroporous,” “macroreticular,” “sponge-like,” and “channeled” have been used, more or less interchangeably, by those skilled in the art to characterize the hazy to completely opaque beads and resins. “Pore-forming,” “phase-separating,” “precipitant,” and “porogen”—even “diluent” less precisely—have all, likewise, been used to refer to the agent used to produce the macroporous structure.
Macroporous resins having a large pore diameter (i.e., 1-10 μm) have also been formed and are described in U.S. Pat. No. 6,323,249. This resin is exceptionally useful in applications where large species are to be isolated. However, the typical low physical strength of these beads, while acceptable for some applications, severely restricts the use of resins formed from these copolymer beads.
Harris has demonstrated the formation of ion exchange resins which form macroporous crosslinked copolymer particles (EP 0168622). However, these resins are similarly not sufficient for many applications. In addition, the formation of these resins is problematic.
Therefore, there is a need in the art for macroporous resins as well as their copolymer precursors having both large pore size and high strength. There is also a need for effective methods to produce these copolymers and resins efficiently and also economically.